# ELECTRONIC SPECTRA OF FULLERENES IN CRYOGENIC RADIO ... ELECTRONIC SPECTRA OF FULLERENES IN CRYOGENIC

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• ELECTRONIC SPECTRA OF FULLERENES IN CRYOGENIC

Inauguraldissertation zur

Erlangung der Würde eines Doktors der Philosophie vorgelegt der

Philosophisch-Naturwissenschaftlichen Fakultät der Universität Basel

von

Mathias Holz aus Finsterwalde, Deutschland

Basel, 2017

Originaldokument gespeichert auf dem Dokumentenserver der Universität Basel edoc.unibas.ch

• Genehmigt von der Philosophisch-Naturwissenschaftlichen Fakultät auf Antrag von

Prof. Dr. John P. Maier

Prof. Dr. Stefan Willitsch

Basel, den 20.09.2016

Prof. Dr. Jörg Schibler

Dekan

ii

• Acknowledgement This little scientific contribution would not have been possible without guidance and support of several individuals.

First of all I would like to thank Prof. John P. Maier who gave me the oppor- tunity to work in his group, as well as for his assistance, financial support and lots of patience throughout my doctoral studies.

I thank Prof. Stefan Willitsch agreeing to be the co-referee of my thesis and Prof. Cornelia G. Palivan for taking responsibility of chairing my defense.

During the whole time I had the privilege to work with Prof. Dieter Gerlich. His deep knowledge about ion-trapping and enthusiasm for science is without equal. It was a fortunate for me and this project to have him participating.

Similar words I could find for Dr. Corey Rice. He became not only a valuable colleague, but even more a very good friend. Although we didn’t work on the same projects he always offered a helping hand to find solutions for any problem. Special thanks also to Dr. Satrajit Chakrabarty not only for explaining me the aspects of the machine when I made my first steps with ion-trapping. I greatly enjoyed the company of Dr. Varun Gupta, Dr. Rainer Dietsche and Dr. Ranjini Dietsche, Kaveh Najafian, Panagiotis Fountas, Karol Filipkowski and Dr. Lindsay Zack. Apart from science we shared plenty of time outside the labs having BBQs by the Rhine, or discussing semi-scientific problems at ”Bester Pizza”. It was really a great pleasure to have met you guys, I hope I will see all of you from time to time.

I’d like to thank Dr. Ewen Campbell working with me in the lab and pushing the machine to its limits to obtain these nice results.

Much of the presented work in this thesis would have never been possible with- out the excellent support of Georg Holderied (electronics), Dr. Anatoly John- son (lasers), Jacques Lecoultre (chemicals, synthesizing), the mechanical workshop with Grisha Martin, Philipp Knöpfel and Dieter Wild. Danni Tischhauser and Maya Greuter are kindly acknowledged for off-scientific matters.

And of course I thank my family and friends in Basel and Berlin for providing support not only in the last five years, but my whole life through. Finally I would like to thank Rebecca for her encouragement, strength and loving care she has been giving.

v

• Abstract Over several decades interest has been devoted to the astronomical enigma of the diffuse interstellar bands (DIBs). These are hundreds of absorption features of interstellar origin seen in the spectra of stars with different strengths and widths spread over the visible and near infrared (NIR). They are typically broader than atomic lines and concluded to be of molecular nature. Polycyclic aromatic hydro- carbons, long carbon-chain molecules, and fullerenes have been suspected as their carriers.

Two of the DIBs showed coincident spectral features recorded in a neon matrix ex- periment for the fullerene C+60. Embedding molecules in a solid matrix are known to induce perturbations of the measured spectrum and consequently, the assignment was classified as tentative. An unambiguous identification of a specific molecule as a carrier can only be made upon measurements of its laboratory gas-phase spectrum under similar conditions as they are present in the interstellar medium. Nevertheless, the recent identification of the infrared signature of C60, C+60 and C70 in the spectra of a protoplanetary and reflection nebula fueled their relevance as possible candidates.

Optical and NIR spectroscopy of large molecules has strong demands on the em- ployed method. Therefore, an existing apparatus was improved and a special spectroscopic technique was thought. The heart of the experiment was a radio- frequency ion trap in which a cryogenic bath of a neutral gas was created to con- fine and prepare the ionic species for further investigations. Electronic gas-phase spectra have been finally obtained by photofragmentation of weakly bound cation- helium complexes, which enabled a confident confrontation with astronomical ob- servations. In the case of C+60, an unequivocal assignment of five DIBs has been achieved, and thus, the first identification of a carrier almost 100 years after their first detection.

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• Contents

Acknowledgement v

Abstract vii

1 Introduction 1 1.1 The Diffuse Interstellar Bands . . . . . . . . . . . . . . . . . . . . . 2 1.2 Fullerenes as Carriers of the DIBs . . . . . . . . . . . . . . . . . . . 3 1.3 Motivation and Thesis Structure . . . . . . . . . . . . . . . . . . . . 5 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Methodology 11 2.1 Ion Motion in Multipole RF-Fields . . . . . . . . . . . . . . . . . . 11

2.1.1 Mass Filtering in a Quadrupole . . . . . . . . . . . . . . . . 12 2.1.2 Ion Trapping . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2 Buffer Gas Cooling of Ions . . . . . . . . . . . . . . . . . . . . . . . 16 2.3 Electronic Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3.1 The Franck-Condon Principle . . . . . . . . . . . . . . . . . 20 2.3.2 Selection Rules . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.4 Spectroscopic Methods . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.4.1 Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . 22 2.4.2 Photodissociation Spectroscopy of Weakly

Bound Complexes . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4.3 Line-Shape Functions . . . . . . . . . . . . . . . . . . . . . . 25

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3 Experimental 31 3.1 Electron Impact Source . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2 Quadrupole Mass Filter . . . . . . . . . . . . . . . . . . . . . . . . 35 3.3 Multipole Radio Frequency Ion Traps . . . . . . . . . . . . . . . . . 37

3.3.1 22-Pole Trap . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.3.2 4-Pole Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.3.3 Piezo Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.4 Ion Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.5 Laser Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.6 Operating procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.6.1 Estimation of Number Densities . . . . . . . . . . . . . . . . 44

ix

• 3.6.2 Ion Trapping and Complex Formation . . . . . . . . . . . . 45 3.6.3 Absorption Cross-Sections . . . . . . . . . . . . . . . . . . . 48

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4 Laser Induced Inhibition of Complex Growth 53 4.1 Proof of Principle on N+2 . . . . . . . . . . . . . . . . . . . . . . . . 55

4.1.1 Experimental Conditions . . . . . . . . . . . . . . . . . . . . 55 4.1.2 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.1.3 Laser Induced Charge Transfer Reaction to Ar . . . . . . . . 57 4.1.4 Dynamical Processes . . . . . . . . . . . . . . . . . . . . . . 58 4.1.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.2 NCCN+–He . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.3 C14H+10–He . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

5 Electronic Spectra of Fullerenes 69 5.1 C+60 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

5.1.1 C+60–He . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 5.1.2 C+60–nHe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.1.3 Comparison to the DIBs . . . . . . . . . . . . . . . . . . . . 79 5.1.4 C+60–L (L =Ne, Ar, Kr, H2, D2, N2) . . . . . . . . . . . . . . 82

5.2 C+70 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 5.2.1 C+70–He . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 5.2.2 Comparison to the DIBs . . . . . . . . . . . . . . . . . . . . 93

5.3 C2+60 –He and C2+70 –He . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5.4 C+84–He . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

6 Conclusion and Outlook 105 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Appendix 109 A Relaxation Kinetics: Microscopic Reversibilty . . . . . . . . . . . . . . 109 B Table Parameters of 9577/9632 DIBs . . . . . . . . . . . . . . . . . . . 111 Bibliography . . . . .

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